ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 66

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Reading the Pin Value
66
ATmega64(L)
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn,
PORTxn} = 0b11), an intermediate state with either pull-up enabled ({DDxn, PORTxn} =
0b01) or output low ({DDxn, PORTxn} = 0b10) must occur. Normally, the pull-up
enabled state is fully acceptable, as a high-impedant environment will not notice the dif-
ference between a strong high driver and a pull-up. If this is not the case, the PUD bit in
the SFIOR Register can be written to one to disable all pull-ups in all ports.
Switching between input with pull-up and output low generates the same problem. The
user must use either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state
({DDxn, PORTxn} = 0b11) as an intermediate step.
Table 25 summarizes the control signals for the pin value.
Table 25. Port Pin Configurations
Independent of the setting of Data Direction bit DDxn, the port pin can be read through
the PINxn Register bit. As shown in Figure 30, the PINxn Register bit and the preceding
latch constitute a synchronizer. This is needed to avoid metastability if the physical pin
changes value near the edge of the internal clock, but it also introduces a delay. Figure
31 shows a timing diagram of the synchronization when reading an externally applied
pin value. The maximum and minimum propagation delays are denoted t
respectively.
Figure 31. Synchronization when Reading an Externally Applied Pin Value
DDxn
INSTRUCTIONS
0
0
0
1
1
SYSTEM CLK
SYNC LATCH
PORTxn
PINxn
0
1
1
0
1
r17
(in SFIOR)
PUD
X
X
X
0
1
XXX
Output
Output
Input
Input
Input
I/O
t
pd, max
Pull-up
0x00
Yes
No
No
No
No
XXX
t
pd, min
Comment
Tri-state (Hi-Z)
Pxn will source current if ext. pulled
low.
Tri-state (Hi-Z)
Output Low (Sink)
Output High (Source)
in r17, PINx
pd,max
2490G–AVR–03/04
0xFF
and t
pd,min